Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of relative height variations between the surface of interest and the reference surface.
A scanning interferometer scans the optical path length difference (OPD) between the reference and measurement legs of the interferometer over a range comparable to, or larger than, the coherence length of the interfering wavefronts, to produce a scanning interferometry signal for each camera pixel used to measure the interferogram. A limited coherence length can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI). A typical scanning white light interferometry (SWLI) signal is a few fringes localized near the zero optical path difference (OPD) position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with Gaussian-shaped fringe-contrast envelope. The conventional idea underlying SWLI metrology is to make use of the localization of the fringes to measure surface profiles.
A common challenge for manufacturers is precise measurement of surface topography. Examples of manufactured items requiring metrology are engine parts, components for magnetic storage devices, flat-panel displays, molded and textured plastic surfaces, mechanical pump surfaces and seals, and minted coins. In these and other Industrial Markets, there is a significant and growing need for fast, accurate metrology of parts having non-flat prismatic surfaces. These parts include three-dimensional (3D) cones, cylinders, and spheres, often having surfaces as small as 2 mm in diameter and 75 mm deep with 3D form tolerances of as low as 0.5 μm. An important example is fuel system valves, which are fundamental building blocks in engines, pumps and other hydraulic systems. Manufacturing the conical form of these parts within tolerance specifications is a high priority. For instance, the roundness of valve seats is important to valve function as it relates closely to leakage—a valve seat not conforming to specified roundness would likely yield a leaky valve. Additionally, many of these surfaces are deeply recessed within narrow cylindrical holes, making precise metrology even more challenging.
Presently, most measurements on fuel system components are mechanical or tactile (e.g., stylus gages). There is a strong interest in this industry to transition to optical techniques, for example by using interferometry, which can improve throughput, data density and uncertainty compared with mechanical techniques. One key advantage of optics is the “3D” aspect of the surface measurement, as opposed to the linear trace of a stylus gage. However, many industrial surfaces such as interior cones are difficult to measure optically, because of their unusual shape and surface texture, when compared to the usual optical testing samples such as mirrors, prisms and lenses.